Shock absorbers for absorbing and dissipating the kinetic energy of impact or shock force are well known. Conventionally destructive shock forces are dissipated by energy absorbing devices such as springs, rubber buffers or hydraulic fluids. Conventional spring and/or elastomeric shock absorbers suffer a disadvantage, however, in that they typically do not dissipate energy at a uniform rate. Conventional shock absorbers deform through the elastic limit of the deforming material resulting in varying rates at which the energy is absorbed. Also the rate of energy dissipation changes with the speed of the crash limiting their predictability. Where hydraulic cylinders are used to control discharge fluids, such as in shock absorbers, fluid is compressed and then released through a controlled outlet/bypass which dissipates the energy. However, on compression the energy transmitted through these devices increases at a geometric rate relative to speed of collapse of the cylinder thus passing the peak energy through the device and dissipating the lower levels of energy at the beginning and end of the crash. Further, heretofore, conventional shock absorbers have achieved mixed results in absorbing and/or dissipating high shock forces, such as those produced by falling elevators, traffic accidents or improvised explosive devices (IEDs).
The applicant has appreciated that a simplified energy dissipation system may be achieved by providing a cutting member and a sacrificial deformation plate or member which on an impact is cut and/or deformed by the cutting member. In a most simplified form, the deformation member is a cylindrical or circular tube of mean diameter (D) and thickness (t) which experiences a stable progressive folding deformation as an efficient energy absorbing member.
Jensen et al. ‘Transition between progressive and global buckling of aluminium extrusions’, Structures Under Shock and Impact VII, Southampton, WIT Press, 2002, 267-277; and “Experimental investigations on the behavior of short to long square aluminium tubes subjected to axial loading”, International Journal of Impact Engineering, 2004 30 973-1003), the disclosures of which are incorporated herein by reference, numerically and experimentally investigated the transition of square aluminum alloy tubes extrusions and found that the energy absorption was dependent on the collapse mode. In particular, total energy absorption decreases when the impact velocity due to inertia forces increased to prevent the global bending of the tube and the early transition from progressive to global bending. An increasing relationship was observed between the L/b and the b/t ratios in the quasi-static tests and impact tests with a velocity of less than 13 m/s, (where b is the width of the extrusion). An inverse relationship was found when the impact velocity was 20 m/s. Thus, the overall response is found to be highly dependent on the location of the first lobes.
Karagiozova and Alves ‘Transition from progressive buckling to global bending of circular shells under axial impact—Part I: Experimental and numerical observations’, International Journal of Solids and Structures, 2004 41 1565-1580, the disclosure of which is incorporated herein by reference, experimental tests on the bending of circular tubes showing that the critical length (Lcr) is influenced by the impact velocity. Numerically, it was observed that circular extrusions made of ductile alloys with a high yield stress and low strain hardening characteristics had a better energy absorption performance than extrusions with a low yield stress and high strain hardening characteristics.
Earlier studies show that given a certain material and particular D/t ratio, and Lcr exists under quasi-static loading. However, for a dynamic loading condition, the collapse mode of an extrusion depends not depend only on material properties, boundary conditions and extrusion geometries, but also on the impact velocity. Furthermore, geometrical imperfections may play a more important role in the dynamic crush conditions.
Galib and Limam Experimental and numerical investigation of static and dynamic axial crushing of circular aluminium tubes', Thin-Walled Struct, 2004 42 1103-1137, the disclosure of which is incorporated herein by reference, describe the crushing of circular aluminum extrusions subjected to variable impact mass and impact velocity values. The progressive folding deformation modes of circular extrusions under both dynamic and static loading are generally the same with the main difference related to the first part of the impact, where a dynamic force is approximately 40-60% higher than a static one. Following this initial high peak force, crush forces oscillated significantly during the formation of the lobes in both loading conditions. The mean dynamic crush forces were about 10% higher than the corresponding values in the quasi-static tests, which indicated the strain rate insensitivity property of this type of material.
With the attempts to control and stabilize the collapse mode and improve the energy absorption capability of the extrusions under axial loading conditions, geometrical discontinuities, due to the easy implementation, are commonly used to initiate a specific collapse mode and improve energy absorption.
Abah et al. “Effects of cutouts on static and dynamic behavior of square aluminium extrusions”, In: Jones, N., Talaslidis, D. G., Brebbia, C. A., and Manolis, G. D., Editors, Structures Under Shock and Impact V, Computational Mechanics, Southampton, UK, 1998, 133-152, the disclosure of which is incorporated herein by reference, describe the effects of circular cutouts at the four edges of square aluminum extrusions, whereby up to 50% reduction of the peak forces was observed depending on the size of the cutout and loading condition, however, the mean crush load remained relatively constant for both loading conditions.
Arnold and Altenhof “Experimental observations on the crush characteristics of AA6061-T4 and T6 structural square tubes with and without circular discontinuities”, Int J Crashworthiness, 2004 9 (1) 829-854), and Cheng and Altenhof “Experimental investigations on the crush behavior of AA6061-T6 aluminium square tubes with different types of through-hole discontinuities”, Thin-Walled Struct, 2006 44 (4) 441-454, the disclosures of which are incorporated herein by reference, describe deformation results for square extrusions with circular discontinuity, or circular, slotted and elliptical holes under quasi-static axial loading. Arnold and Altenhof and Chen and Altenhof, supra, show that a significant reduction of the initial peak crush load and higher crush force efficiency (CFE) of the extrusions. Furthermore, energy absorption capacity may be improved by altering the deformation mode within the extrusion, through the implementation of the geometrical discontinuities within the tubular member.
While control of the collapse mode of the extrusions is reliable under the quasi-static axial loading through the implementation of initiators, the initiation of the desired deformation mode under impact loading becomes more complicated and may lead to a very poor energy absorption mode since the collapse mode greatly depends on the impact velocities. The applicant has appreciated that this collapse mode dependency of extrusions can be eliminated when for example a cylindrical tubular member is configured to experience a splitting or cutting deformation mode. Reddy and Reid “Axial splitting of circular metal tubes”, Int J Mech Sci, 1986 28 (2) 111-13), the disclosure of which is incorporated herein by reference, describe the splitting of mild steel and aluminium circular tubes under both quasi-static and dynamic axial loading conditions using a mandrel type of die. Crack formations were observed for all tests with or without pre-slits on extrusions. The cracked extrusions then generated strips which curled afterwards. A peak load was observed along with the formation of the splitting mode and a steady crush force was also observed during the steady state splitting process. Such a mode of deformation illustrated a stroke efficiency greater than 90%, although it is not as efficient as a tube undergoing axial crush or inversion.
Huang et al. [11-12] (Huang, X., Lu, G and Yu, T. X, ‘Energy absorption in splitting square metal tubes’, Thin-Walled Struct, 2002 40 (2) 153-165 and ‘On the axial splitting and curling of circular metal tubes’, Int J Mech Sci, 2002 44 (11) 2369-2391), the disclosure of which is incorporated herein by reference, investigated the quasi-static axial splitting behaviour of circular and square mild steel and aluminium tubes using a conical die. Three stable energy dissipation mechanisms were reported, namely, a ‘near tip’ tearing and splitting of the tube, a ‘far-field’ plastic bending and stretching of curls, and a dissipation mechanism associated with friction due to the interaction of the tube with the die.
Jin et al. “An experimental investigation into the cutting deformation mode of AA6061-T6 round extrusions”, Thin-Walled Struct, 2006 44 (7) 773-786, the disclosure of which is incorporated herein by reference, describes cutting deformation of cylindrical aluminum (AA6061-T6) extrusions under a quasi-static loading condition using a cutter. The cutting deformation mode of the circular extrusion was observed to be very stable and controllable. An extremely high crush force efficiency of 95% and a constant steady-state cutting force were reported, which led to ideal energy absorption characteristics. The cutting process was identified as clean cut due to the observed Load/displacement response.
Majumder et al. ‘Quasi-static axial cutting of AA6061-T4 and T6 round extrusions’, IMechE Part L: J. Materials: Design and Applications, 2008 222 183-195, the disclosure of which is incorporated herein by reference, describes cutting deformation behaviour of circular AA6061-T6 and T4 extrusions under quasi-static loading conditions with two different extrusion wall thicknesses (t) of 3.175 mm and 1.587 mm using cutters described in Jin, supra. It was observed that T6 temper extrusions with both wall thicknesses and T4 temper extrusion with t=3.175 mm exhibited a clean cut, while T4 temper extrusion with t=1.587 mm showed braided cut. The steady state force reduced approximately 50% when the extrusion wall thickness reduced 50% for both temper extrusions. The cutting deformations were observed to be very stable and repeatable.
Jin and Altenhof ‘Experimental observations of AA6061-T6 round extrusions under a cutting deformation mode with a deflector’, Int. J. Crashworthiness, 2008 13 (2) 127-138, the disclosure of which is incorporated herein by reference, further describes cutting deformation behaviour of circular AA6061-T6 extrusion with the presence of curved and straight deflectors. In particular, it is shown that the CFE decreased from 95% to 81% and 68% with the presence of the curved deflector and straight deflector respectively. A constant cutting force was observed after the extrusion petalled sidewalls contacted the deflector and bent outwards. The cutting deformations with the deflector appeared to be stable and controllable.
Jin et al. “Axial cutting of AA6061-T6 circular extrusions under impact using single- and dual-cutter configurations', In Press: International Journal of Impact Engineering, 2009, doi:10.1016/j.ijimpeng.2009.01.003, the disclosure of which is incorporated hereby reference further describes energy absorption behaviour of circular AA6061-T6 extrusions under dynamic and quasi-static loadings using single- or dual-cutter and a deflector assembly. It was observed that the total energy absorption of the extrusion experienced by the single cutting deformation was typically less compared to the progressive folding deformation, but much greater than the global bending deformation. The total energy absorption of the extrusions experienced by the dual stage cutting mode surpassed the progressive folding mode. Dual stage cutting is typically a superposition of two single stage cutting processes with a displacement delay, on the increased force due to the second series of cuts, approximately equal to the cutter thickness.